It is generally known to provide an engine that can be powered by various non-chemical and mechanical means. For example, thermal differences may be used to power an engine to provide the thermal dynamic sources producing mechanical and electrical power through an alternator. The thermal dynamic engines include various thermal dynamic cycles which are harnessed to provide the mechanical energy for various engines. Various cycles include Stirling cycles, brayton cycles, and rankine cycles. These various cycles can be employed in engines using the same or similar name as the engine.
Generally, each of these engines provide for producing energy from one of the related thermal dynamic cycles. The thermal dynamic cycles and the related engines require a differential in thermal energy to create the mechanical and electrical energy from the engine. Nevertheless, controlling and making efficient various engines of the thermal dynamic cycles is difficult and requires precise treating and operation.
For example, a Stirling cycle engine is a thermal energy to a mechanical energy conversion device that uses a piston assembly to divide a fixed amount of gas between at least two chambers. The chambers are otherwise connected by a gaseous fluid passage equipped with a heat source, recuperation, and heat sink exchangers. The piston assembly has at least two piston heads that are separated and act on both chambers simultaneously through mutual coupling. As the volume in one chamber is increased, the volume in the other volume decreases and vice versa, although not strictly to the same degree since one of the piston heads may have a greater area than the other piston head by design. The movement of the piston assembly in either direction creates an elevation of pressure in the chamber that experiences a decrease in volume while the other chamber experiences an increase in volume and decrease in pressure. The pressure differential across the two chambers decelerates the pistons, and causes a flow of gas from one chamber to the other, through the connecting fluid passage with its heat exchangers. The heat exchangers tend to either amplify or accentuate the gas volume flowing through them, depending on whether the gas is either heating or cooling as it flows through the fluid exchange. The fluid exchange, also a regenerator heat exchanger, stores heat from the hot end gas as it flows to the cool end. Likewise the regenerator gives up heat to the cooler gas coming from the cold end. This improves the efficiency of the thermal cycle.
The character of the piston assembly as a finite massive moving object now comes into play according to the laws of motion and momentum. The piston will overshoot the point at which the pressure forces across the piston are in balance. Up to that point, the piston has had an accelerating pressure differential force that charges it with kinetic energy of motion. Once the net forces on the piston balance, the acceleration ceases, but the piston moves on at its maximum speed. Soon the pressure differential reverses and the piston decelerates, transferring its kinetic energy of motion into gas pressure/volume energy in the chamber toward which the piston has been moving up to this point. The increased pressure in the chamber now accelerates the piston in the opposite direction to the point where it reaches its maximum velocity in the opposite direction at the force balance point, and then decelerates as an increasing pressure differential builds in the other chamber. Once again, the piston stops, reverses direction, and repeats the process anew. This is a case of periodic motion as the energy is passed from the form of kinetic energy in the piston assembly to net pressure/volume energy in the chambers.
The periodic motion tends to be damped by small irreversibilities, especially the gas that is pumped back and forth from one chamber to the other through the fluid passage. This is the normal case for a Stirling engine in an isothermal state. However, when it is thermally linked to hot source and cool sink reservoirs at the source and sink heat exchangers respectively, the gas flowing into one of the chambers is heated while the gas flowing into the chamber on the other side is cooled. In this way, a given mass of pressurized cool gas sent to the hot chamber is heated and amplified in volume to a sizable shove. Conversely, a given mass of hot gas leaving the hot side chamber is reduced in volume as it is cooled by passage through the heat exchangers, and the cooled gas push in the cool side chamber is thereby attenuated dramatically due to the reduced volumetric flow of the cooler gas. Thereby, a change in the piston position, and its affects on gas temperature and pressure within the Stirling cycle engine, cause portions of the hot reservoir thermal energy to turn into periodic mechanical piston energy and gas pressure/volume energy, and the remaining thermal energy to flow to the cool reservoir in periodic fashion.
The compressible gas within the two chambers and the piston moving between those chambers form a spring-mass system that exhibit a natural frequency. Similarly, the motion of gas between the two chambers has its own natural frequency of a lower order. The conversion of thermal energy to mechanical within this system would cause such a system have successively higher amplitudes until mechanical interference or some other means of removing the energy appears. For many commercial Stirling cycle heat engine systems, a power piston operating at the same frequency, but out of phase with heat engine piston, is used to remove the excess mechanical energy and convert it into useful work.
One way to produce this energy conversion is to use the time varying position of the power piston to produce a time varying magnetic flux in an electrical conductor. This produces an electromotive potential which can be consumed locally, or remotely over transmission lines, by connection to an electrical appliance such as a motor, battery charger, or heater. Commonly, this is done by using the power piston to drive an alternator mover through a mechanical link. The alternator mover is what converts a time varying position within the alternator into time varying magnetic flux in the alternator electrical conductor(s).
Stirling cycle engines can be designed and tuned for optimal efficiency at various different temperatures for the source heat exchanger. Nevertheless, once a Stirling cycle engine is tuned or optimized for particular operating conditions its efficiency dramatically decreases when these optimum conditions are not maintained. If the concentrated sunlight entering the absorber cavity varies slightly, the efficiency of the single Stirling cycle engine can be compromised. Such variations can occur when only a slight haze or foggy condition exists between the concentrator and the sun. Moreover, time of day and seasonal variations can cause the sunlight to travel through more or less atmosphere and effect the insolation, thereby adversely affecting the concentrated solar power level to a value that is not consistent with operating the Stirling cycle engine at its optimum efficiency.
When the insolation becomes too low, the Stirling engine overcools the thermal cavity. At this point, the temperature of the thermal cavity is below the design temperature of the Stirling engine. This will result in a reduction in the heater head temperature causing the engine to operate at a lower efficiency point. Although, the design of the Stirling engine can be modified by adjusting the stroke length to partially compensate for this, the Stirling engine still may not operate at optimum or designed conditions. Therefore, over a long period of time, this inefficiency can have a significant impact on the life cycle cost of the units of energy produced.
It is desirable to provide a system and method that can automatically measure and tune a thermal dynamic engine, for example a Stirling cycle engine, to produce an optimal operation of the engine. Moreover, the system may diagnose various inefficiencies that are created over the life of the engine and can compensate for such inefficiencies to alter the operation of the Stirling engine to provide for an optimal energy transfer from the Stirling cycle engine. Moreover, it is desirable to provide a system which can finally control the Stirling cycle engine to provide for optimal power transfer over various portions of the life cycle, such as start-up and maintenance of the Stirling cycle engine. Also, the reduction of heat flux to the hot end reduces the temperature and available output power. By reducing the stroke of the Stirling, the power transfer is reduced and the hot end temperature will rise, improving the efficiency by maintaining the optimal or tuned temperature of the engine.